Package org.apache.struts2.inject.util

Class ReferenceMap<K,V>

java.lang.Object

org.apache.struts2.inject.util.ReferenceMap<K,V>

All Implemented Interfaces:

Serializable, Map<K,V>

Direct Known Subclasses:

ReferenceCache

public class ReferenceMap<K,V>
extends Object
implements Map<K,V>, Serializable

Concurrent hash map that wraps keys and/or values in soft or weak references. Does not support null keys or values. Uses identity equality for weak and soft keys.

The concurrent semantics of ConcurrentHashMap combined with the fact that the garbage collector can asynchronously reclaim and clean up after keys and values at any time can lead to some racy semantics. For example, size() returns an upper bound on the size, i.e. the actual size may be smaller in cases where the key or value has been reclaimed but the map entry has not been cleaned up yet.

Another example: If get(Object) cannot find an existing entry for a key, it will try to create one. This operation is not atomic. One thread could put(Object, Object) a value between the time another thread running get() checks for an entry and decides to create one. In this case, the newly created value will replace the put value in the map. Also, two threads running get() concurrently can potentially create duplicate values for a given key.

In other words, this class is great for caching but not atomicity.

Author:

crazybob@google.com (Bob Lee)

See Also:

• Serialized Form

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
protected static interface	ReferenceMap.Strategy

Constructor Summary

Constructors

Constructor	Description
ReferenceMap(ReferenceType keyReferenceType, ReferenceType valueReferenceType)	Concurrent hash map that wraps keys and/or values based on specified reference types.

Method Summary

Modifier and Type	Method	Description
void	clear()
boolean	containsKey(Object key)
boolean	containsValue(Object value)
Set<Map.Entry<K,V>>	entrySet()	Returns an unmodifiable set view of the entries in this map.
V	get(Object key)
protected org.apache.struts2.inject.util.ReferenceMap.PutStrategy	getPutStrategy()
boolean	isEmpty()
Set<K>	keySet()	Returns an unmodifiable set view of the keys in this map.
V	put(K key, V value)
void	putAll(Map<? extends K,? extends V> t)
V	putIfAbsent(K key, V value)
protected ReferenceMap.Strategy	putIfAbsentStrategy()
protected ReferenceMap.Strategy	putStrategy()
V	remove(Object key)
boolean	remove(Object key, Object value)
V	replace(K key, V value)
boolean	replace(K key, V oldValue, V newValue)
protected ReferenceMap.Strategy	replaceStrategy()
int	size()
Collection<V>	values()	Returns an unmodifiable set view of the values in this map.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface java.util.Map

compute, computeIfAbsent, computeIfPresent, equals, forEach, getOrDefault, hashCode, merge, replaceAll

Constructor Details

ReferenceMap

public ReferenceMap(ReferenceType keyReferenceType,
 ReferenceType valueReferenceType)

Concurrent hash map that wraps keys and/or values based on specified reference types.

Parameters:

keyReferenceType - key reference type

valueReferenceType - value reference type

Method Details

get

public V get(Object key)

Specified by:

get in interface Map<K,V>

put

public V put(K key,
 V value)

Specified by:

put in interface Map<K,V>

remove

public V remove(Object key)

Specified by:

remove in interface Map<K,V>

size

public int size()

Specified by:

size in interface Map<K,V>

isEmpty

public boolean isEmpty()

Specified by:

isEmpty in interface Map<K,V>

containsKey

public boolean containsKey(Object key)

Specified by:

containsKey in interface Map<K,V>

containsValue

public boolean containsValue(Object value)

Specified by:

containsValue in interface Map<K,V>

putAll

public void putAll(Map<? extends K,? extends V> t)

Specified by:

putAll in interface Map<K,V>

clear

public void clear()

Specified by:

clear in interface Map<K,V>

keySet

public Set<K> keySet()

Returns an unmodifiable set view of the keys in this map. As this method creates a defensive copy, the performance is O(n).

Specified by:

keySet in interface Map<K,V>

values

public Collection<V> values()

Returns an unmodifiable set view of the values in this map. As this method creates a defensive copy, the performance is O(n).

Specified by:

values in interface Map<K,V>

putIfAbsent

public V putIfAbsent(K key,
 V value)

Specified by:

putIfAbsent in interface Map<K,V>

remove

public boolean remove(Object key,
 Object value)

Specified by:

remove in interface Map<K,V>

replace

public boolean replace(K key,
 V oldValue,
 V newValue)

Specified by:

replace in interface Map<K,V>

replace

public V replace(K key,
 V value)

Specified by:

replace in interface Map<K,V>

entrySet

public Set<Map.Entry<K,V>> entrySet()

Returns an unmodifiable set view of the entries in this map. As this method creates a defensive copy, the performance is O(n).

Specified by:

entrySet in interface Map<K,V>

putStrategy

protected ReferenceMap.Strategy putStrategy()

putIfAbsentStrategy

protected ReferenceMap.Strategy putIfAbsentStrategy()

replaceStrategy

protected ReferenceMap.Strategy replaceStrategy()

getPutStrategy

protected org.apache.struts2.inject.util.ReferenceMap.PutStrategy getPutStrategy()